Depending on the situation the desired product can be the permeate, thus the gas that is passed through the membrane wall, or the retentate, thus the gas that has not passed through the membrane wall.
Traditionally such a device for separating mixtures of gases comprises a compressor installation in which the pressure of the feed gas, which in reality is a gas mixture, for example air, is increased, and a membrane installation in which the feed gas is separated into a permeate gas and a retentate gas, for example oxygen and nitrogen.
Because selective membranes are very expensive and constitute a large proportion of the total costs of such an installation, because the capacity of such an installation is approximately proportional to the pressure, and because the energy required for the compression of the feed gas increases much less than linearly with the pressure, the total cost of the operation of such a device can be reduced by increasing the operating pressure to substantially above the desired product gas pressure.
In order to keep this operating pressure constant, such a device is generally equipped with a minimum pressure valve, which, independent of the offtake and thus the product gas pressure by the consuming installation, keeps the operating pressure in the membrane unit at the desired level.
If the required product gas flow of such a device is less than the nominal design flow, the product purity is higher than the design purity and the recuperation level falls, i.e. the product gas flow/feed gas flow ratio, at least for devices in which the retentate gas is the desired product.
This is disadvantageous because too pure a product gas can be technically undesirable in the application, and because the processing of an unnecessarily high feed rate brings about unnecessary costs.
Methods and devices to improve this are described for example in EP 1.324.815, U.S. Pat. No. 5,496,388, U.S. Pat. No. 5,649,995 and DE 10.2010.003.507.
EP 1.324.815 describes a device and an accompanying method in which a speed controlled compressor is used as a compressor installation, which adjusts the speed of the compressor on the basis of a gas sensor in the product gas, or possibly another sensor that yields a value that is characteristic of the product gas purity.
This has the disadvantage that this method only works with speed controlled compressors and not with compressor types that are controlled in another way.
As a result of this, if the compressor is also operating at its minimum capacity and the product gas flow decreases further, the desired product purity is not obtained but a product purity that is too high.
A further disadvantage is that the speed of a compressor cannot just be adjusted without the risk of damage or requiring frequent maintenance. This speed range is not necessarily known, or its importance is not necessarily recognised by the designer of a membrane installation, such that there is a risk of premature failure of the compressor. This is all the more so because the acceptable speed range can be a complex function of other parameters of the compressor, such as the operating pressure.
Another disadvantage is that the solution of EP 1.324.815 is not a solution if the compressor installation, in addition to supplying the membrane unit, is also a supply for another installation, with its own requirement, such as a compressed air network.
U.S. Pat. No. 5,496,388 describes a device in which a screw compressor is used, that is equipped with a slide valve at the level of the screws, and which, if certain conditions are satisfied, is controlled on the basis of a measuring signal from a sensor that measures the purity of the product gas, such that the capacity of the feed gas is controlled.
This solution also has the disadvantage that it is not a solution if the compressor installation, in addition to supplying the membrane installation, also supplies another installation, with its own requirement, such as a compressed air network.
A further disadvantage is that this adjustment and method can only be applied to compressors that are equipped with a slide valve.
U.S. Pat. No. 5,649,995 describes a device in which the purity of the product gas is controlled, not by controlling the compressor, but by constructing the traditional known minimum pressure valve in the retentate gas outlet as a control valve, which is controlled on the basis of the purity of the product gas measured by a purity sensor, in order to obtain a constant purity of product gas in this way.
A disadvantage of this is that the product gas flow cannot be hereby controlled.
A lot of compression energy is hereby lost because the compressor is always operating at full capacity, thus supplying a large flow of gas at high pressure, which then generally has to be expanded.
In DE 10.2010.003.507 a reduction of the gas supplied to the membrane unit is effected by placing a control valve between the compressor and the membrane unit, which adjusts the inlet flow on the basis of a parameter of the product gas, for example purity or pressure, whereby the compressor is controlled via an on-off control, and pressure variations therein are accommodated by a buffer volume placed directly after the compressor.
The traditional devices, including the said improvements, all have the disadvantage that if the offtake of product gas is greater than the design capacity of the installation, the purity becomes lower than the design value. This can lead to hazardous situations for example when nitrogen with a low oxygen content is produced to prevent explosions.
Moreover the improvements do not prevent the problem of too high a product gas purity when the product gas offtake is low.
The said controls of compressors are also complex or cannot be constructed when compressors other than the said specific types of controllable compressors are used, and certainly not when different types of compressors are used in one compressor installation.
Also the known solutions to control the purity do not take account of important aspects of the operation of compressors, such as minimum energy consumption, maximum lifetime and optimum maintenance intervals, which are all greatly influenced by the operating conditions of a compressor.